1. Field of the Invention
The present invention relates to an improved wet cleaning approach to back-end of line (BEOL) processes. More particularly, the present invention relates to an improved two-step wet cleaning process incorporated with an extrusion inhibition step for post-etch copper structures, particularly, for copper-dual damascene structures. The two-step wet cleaning process comprises a mild oxidation step followed by an oxide etch step. This approach demonstrates clean copper dual damascene structures having no copper extrusions that arise due to oxidation-reduction (“redox”) reactions in a wet cleaning solution abundant in dissolved copper ions.
2. Description of the Prior Art
Aluminum alloys with SiO2 dielectrics have been the materials of choice for interconnect systems since the dawn of the integrated circuit (IC) era. These materials were convenient to process using mature subtractive etch processes for metal line patterning. However, as ICs have relentlessly marched down the path towards smaller geometry in the pursuit of increased speed and integration density, the traditional Al/SiO2 interconnect system has shown itself to be a limiting factor. Cu-dual damascene architectures with low-k dielectrics are thus developing and becoming the norm now in forming interconnects in the BEOL processes.
Although Cu/low-k materials present many benefits from a circuit perspective, they require an entirely new process technology for interconnect fabrication. Fortunately, many of these process issues have been addressed with intermediate interconnect systems employing Cu metal with low-k dielectrics. Some of these low-k materials include FLARE™, SiLK™ and BCB (porous dielectrics). This Cu/low-k material process flow led to the development of Cu dual damascene processes in order to overcome the difficulties presented with reactive ion etching (RIE) of Cu. However, processing with these Cu/low-k material systems presents an entirely new set of problems that must be overcome. Etching vias and trenches results in the formation of complex organometallic polymers, which are difficult to clean using dry processes. Since Cu does not possess the self-passivating properties of Al, the tendency of Cu to corrode when exposed to an oxidizing environment is also a major concern. This is especially problematic with the wet immersion processes required for cleaning Cu/low-k etch residues.
One approach typically used to clean copper structures after via/trench dry etch is using a very diluted aqueous HF-based cleaning process. An alternative approach featuring its effectiveness, which was developed by Mattson Technology Wet Process Division (Exton, Pa.) and United Microelectronics Corp. (Hsinchu, Taiwan), includes a two-step process based on the diluted HF-based cleaning process. The first step of the two-step process is a mild oxidation step consisting of a dilute H2O2 solution 36:1 and a surfactant with megasonics irradiation. This step removes some polymer residues and the sputtered Cu on the sidewalls of the vias and trenches, and oxidizes the copper surface. The first step can be used without a surfactant. The second step is a mild oxide etch utilizing dilute HF, NH4F or NH2OH. This step undercuts the polymer residues and removes both leftover sputtered copper from the sidewalls, and oxide, such as CuOx and Cu(OH)2, from the copper surface.
Referring now to FIG. 1 of a post-etch dual damascene structure 30 formed on a silicon substrate 10, in an acidic ambient, such as HF solution, the lift CuOx and Cu(OH)2 molecules dissolve and thus produce massive copper ions (Cu2+) in the solution (see eq.1). In FIG. 1, after dipping the silicon substrate 10 into an acidic oxide etch solution for a certain time period, a recess 13 formed due to the Cu loss is observed at a top surface of a metal-1 (M-1), i.e. first level metal, Cu wiring line 22 which is electrically connected with a P+ diffusion region 12 of the silicon substrate 10 via a tungsten plug 16. In the meantime, an undesirable extrusion 15 is formed atop an adjacent M-1 Cu wiring line 24 that is electrically connected with a N+ diffusion (electron-rich) region 12 of the silicon substrate 10 via a tungsten plug 18. A reasonable explanation for this phenomenon is that the P-N junction fabricated in the silicon substrate 10 provides an electrical path for electrochemical reactions. Excessive copper ions deposit on the anode (i.e. N+ region connected Cu wiring line) due to reduction reaction (see eq.2). In the reduction of cupric oxide the oxidation number of copper has changed from +2 to zero by the gain of two electrons